(a) Field of the Invention
The present invention relates to an apparatus for measuring electrical impedance correctly. More specifically, the present invention relates to an apparatus for measuring electrical impedance applicable for a reliable analysis of body fat. However, the apparatus according to the present invention can be applied to other fields apart from analysis of body fat.
The apparatus for measuring electrical impedance includes a device applying electrical current to a subject and a difference amplifier measuring difference voltage induced to the subject. The apparatus for measuring electrical impedance can be applied in various fields. For example, body fat analysis can be done conveniently by measuring electrical impedance with a relatively simpler apparatus than that for other measurements.
(b) Description of the Related Art
In many measures of electrical impedance such as can be used in body fat analysis, electrical impedance is measured by using 4-electrode method. To measure body impedance by 4-electrode method AC is applied to body through a pair of electrodes contacted to body, and a difference amplifier measures voltage difference between another pair of electrodes contacted to other parts of the body. The reason for using the 4-electrode method is to reduce the effects of the contact impedance between the electrodes and human skin. For analysis of segmental body fat, at least two pairs of electrodes are used to measure each segment, and a multiplexer can change the measuring pairs of electrodes. However, various errors can occur at high frequency in an apparatus for measuring electrical impedance. Particularly, it is not easy to measure body impedance correctly at high frequency.
One of the representative errors occurring in measurement of the electrical impedance is due to a common mode voltage inputted to a difference amplifier. A common mode voltage means an average value of two voltages inputted to a difference amplifier.
Another error is due to a limited input impedance of a preamplifier. The preamplifier is an amplifier located as close as possible to an electrode for measuring voltage, and is referred as an input buffer or front end. To extract more useful information from electrical impedance measure, it is necessary to measure impedance in a wide range from low frequency to high frequency. However, the two errors increase as frequency rises.
In analyzing electrical impedance for body, since electrode impedance is very large, a common mode voltage inputted to a difference amplifier is increased remarkably. In case that common mode voltage is high, a voltage outputted from a difference amplifier is incorrect. Thus, it is necessary to reduce a common mode voltage inputted to a difference amplifier for accurate measurement of electrical impedance, and a common mode feedback is used for the reduction of the common mode voltage (Rosell, J., and Riu, P.: ‘Common-mode feedback in electrical impedance tomography’, Clin. Phys. Physiol. Meas., V. 13(Suppl. A.), pp. 11–14, 1992). However, a conventional common mode feedback method has a problem of oscillation at a high frequency. Therefore, it is difficult to apply a common mode feedback method at a high frequency over tens of kHz.
An input impedance of a preamplifier at high frequency is determined by an input capacitance present at input terminal of the preamplifier. The preamplifier is commonly constructed by using an operational amplifier, and input capacitance can be several pF. The impedance at 1 MHz corresponding to the capacitance of such magnitude is several tens of kΩ. Thus, a loading error occurs, since input impedance of the preamplifier is not high enough compared to that of an electrode. Thus, for the accurate measurement of electrical impedance, it is necessary to have input impedance increased by reducing input capacitance of the preamplifier.
In a negative capacitance circuit (FIG. 3) known as a conventional method for reducing input capacitance, a positive feedback of signal outputted from a preamplifier is applied to input terminal of the preamplifier through a capacitor. (Rigaud, B., Record, P. M., Anah, J., and Morrucci, J. P.: ‘Active electrodes for electrical impedance tomography: The limitation of active stray capacitance compensation’, Annual Int. Conf. of the IEEE Eng. in Med. and Biol. Soc., V. 13, pp. 1587–1588, 1991.)
In the conventional method a negative capacitance obtained from the positive feedback cancels out the input capacitance of the preamplifier, and input capacitance of the preamplifier becomes zero.
However, as a frequency gets high, a loop gain becomes high, which causes a risk of oscillation. Further, a loop gain depends on impedances of electrodes. Therefore, an operational amplifier of narrow bandwidth should be used to suppress oscillation in the conventional method, making it difficult to select an operational amplifier with adequate bandwidth considering impedance of electrode.
Further, it is difficult to change a gain margin when impedance of electrode is changed. And if a preamplifier has gain peaking generated at a frequency of maximum loop gain, the gain margin corresponding to gain peaking vanishes, which results in easy oscillation.
In sum, an apparatus for measuring electrical impedance causes an error by a common mode voltage and another error by an insufficient impedance of a preamplifier. This calls for an apparatus which can reduce the errors by a common mode voltage and by an insufficient impedance of a preamplifier.